The United States is currently tied to energy derived from petroleum resources. According to the Energy Information Administration (EIA), 68% of petroleum oil consumed by the U.S. is used to fuel our nation's transportation needs. Throughout much of the twentieth century the U.S. was able to depend on ample domestic supplies of petroleum, however, domestic oil production in the contiguous states peaked in 1970 and has been declining ever since. The U.S. economy relies heavily on diesel-powered vehicles for transportation of people and goods and diesel fuel constitutes more than 25% of the nation's total fuel use. Diesel engines provide the power to move 94% of all freight in the U.S. as well as 95% of all transit buses and heavy construction machinery. The nation consumes more than 90,000 gallons of diesel fuel every minute.
More than 66% of the nation's diesel fuel is refined from imported oil and this percentage is growing at a rate of 1% per year. There is a need for the U.S. to develop renewable alternatives to diesel fuel to diversify the available alternatives to petroleum fuels for transportation. Biodiesel is such a renewable and domestically produced diesel fuel alternative that directly displaces petroleum diesel fuel. Biodiesel is a very desirable energy source because it is the most effective liquid fuel use of the abundant natural resource, sunlight, as evidenced by its excellent energy balance—biodiesel yields 3.2 units of fuel product energy for every unit of fossil energy consumed in its life cycle. Biodiesel can be produced from any triglyceride oil and may be blended with diesel fuel in any proportion.
Biodiesel is most often produced by transesterification, which exchanges the alkoxy group of an ester compound by another alcohol in the presence of acid or base catalyst. The transesterification process in biodiesel production is the reaction of a triglyceride with an alcohol to form esters (the biodiesel fuel) and glycerol (a by-product). The most common form of biodiesel is methyl esters of long chain fatty acids, although ethyl ester biodiesel exits as well. The biodiesel esters produced in this fashion can be injected as fuels into diesel engines either pure or blended with fossil diesel.
The transesterification reaction may be catalyzed by either acids or bases, and the base-catalyzed route is the most popular because of the reaction efficiency and mild operating conditions. The base catalyst used for transesterification of the oil to produce biodiesel commercially is typically sodium hydroxide or potassium hydroxide. A mixture of alcohol and the base catalyst is prepared and added to the oil or fat. Excess alcohol is normally used to ensure total conversion of the fat or oil into the corresponding esters. Care must be taken to monitor the amount of water and free fatty acids in the incoming oil or fat. If the free fatty acid level or water level is too high it may cause problems with soap formation and the separation of glycerin/glycerol by-products downstream.
After the reaction, the unreacted methanol, or ethanol, and the catalyst must be removed to purify the methyl ester. Reaction time for conventional batch processes of making biodiesel typically ranges from 1 to 8 hours, and separation time for contaminant removal adds another 8 to 16 hours. The time and expense of this further processing is the primary reason that conventionally produced biodiesel fuels are typically not cost competitive with petroleum diesel fuel. The production cost of most biodiesel fuels is more than 1.5 times greater than that of petroleum derived diesel. In addition to the expense, the production and wash cycles generate many gallons of waste water that must be treated in onsite sewer plants or in publicly owned treatment works. This waste water treatment limits where the plants can be built. As a result, production levels of biodiesel are determined by the number and size of tanks used within a given facility, as well as available land area.
Accordingly, there is a need in the industry for biodiesel production methods that reduce the time and costs of the esterification and decontamination processes while increasing the production of useable byproducts, such as animal feeds and fertilizer. Preferably, these processes would increase the yield and purity of the biodiesel product while reducing the production of wastewater and toxic or environmentally harmful byproducts. Additionally the processes should reduce the time and space requirements below those needed for the traditional processing means.